Calcium silicate hydrate (C-S-H) is the main hydration product of Portland cement that mainly contributes to concrete's physical and mechanical properties. The porosity of C-S-H plays a vital role in the durability-based performance of concrete. Most of the information about the microstructure of C-S-H now is only in 2D due to the complex morphologies of hydration products. This dissertation aims to investigate synthetic C-S-H and C-A-S-H with various Ca/Si ratios and curing temperatures using Transmission Electron Microscope (TEM) tomography. The 3D reconstructions of all samples were done using the simultaneous iterative reconstruction technique (SIRT). The highest beam dose that doesn’t damage the samples for TEM tomography is 5000 e-/(nm2 s) for all samples. The 3D reconstructions and selected ROI analysis confirm that there are only foil-like structures for all C-S-H and C-A-S-H samples. Both CPSD and CMIP reveal that most of the pore sizes of all samples are below 40 nm. However, C-S-H at Ca/Si ratio 1.0 and 1.6 seem to have pore sizes larger than 100 nm. The curing temperature causes a lower porosity in the structure of both C-S-H and C-A-S-H. However, the porosity is not affected by chemical composition. Scanning transmission electron microscopy (STEM) is used to compare the results with TEM. The 3D results of both TEM and STEM tomography show no significant difference. Still, the continuous pore size distribution with CMIP simulations shows a remarkable difference. C-S-H at Ca/Si ratio 1.0 from TEM tomography has 3 dominant pore sizes at 22.2, 28.8 and 37.7 nm, while STEM tomography has only one dominant pore size at 6.0 nm. The pore network’s size of TEM_C-S-H 1.0 is significantly larger but less dense than STEM_C-S-H 1.0. A preliminary study of a fluid flow model has been accomplished. Both porosity and permeability increase with samples higher Ca/Si ratio, and higher equilibrated temperatures. The successful model will enhance the understanding of the relationship of mechanical properties and porosity of cement and concrete. TEM tomography combine with Xlib plugin in ImageJ shows the capability to determine the 3D nanostructure and pore networks of C-S-H and C-A-S-H. To determine the sustainability of cements and concrete, the understanding of the relationship of mechanical properties and porosity is needed. The resulting information of 3D nanostructure and pore analysis provide these critical information for developing a new generation of optimized green cement and concrete.